In the most scientific way possible? How real is the possibility that there might actually be alternative universes?

As the others here are implying, the current consensus in the Physics community is that the many worlds hypothesis is not falsifiable, and so should be disregarded in favor of funding theories that can be proven or disproven. String theorists, the biggest proponents of the many worlds theory, are generally ostracized from greater physics academia since they have failed to produce any experimental designs to validate their claims for several decades after taking up a lot of tenured positions in the 90's, along with the "pop science" PR campaigning for their position that started then with some popular books and continues to this day in the works of Michio Kaku for example.

Relevant panels by leading physicists here: https://youtu.be/CNjckutR7BE?si=t88BigaInz01zamL
and here: https://youtu.be/eOvqJwgY8ow?si=5pbtgQqByJfi0Opi

Scientifically, we cannot yet talk about theories that are untestable and unfalsifiable. Sure, math or some other theory may imply other dimensions or parellel universes, but how will you measure one?

Would a ruler suffice?
- Oof sorry. Gonna need a protractor.
Scientifically, there are no theories that are untestable and unfalsifiable.
- How can you test/falsify the existence of something you can't interact with in any way?
- There are a great many theories that are untestable and unfalsifiable. The existence of a God or a Creator is a hotly debated one, for instance.
If your mathematical model tells you something should exist, doesn't that necessarily mean that it's something you can interact with, and thus measure given sufficiently advanced technology?
- Perhaps but it's still not testable at that point.
Interestingly, parts of this universe are right now becoming unmeasurable, un observable, unfalsifiable to us even now. With dark energy propelling the expansion of space at an increasing rate, far-away regions of space can be moving away from us faster than light. This means they’re no longer part of our observable universe - we’ll never be able to reach them, see light from there, interact with these places in any way. Not the same universe anymore.
Someday living beings in the Milky Way might believe it’s the only galaxy that exists because no others will be observed. If one of them postulates that maybe there are all these other galaxies that are just out of reach - pinched off from our universe - in parallel universes you might say…. if someone says all that, it could also be just as handily dismissed.

In the most scientific way possible, there's no way these could affect our own. If they did, we'd consider that mechanism part of our universe. Real possibility - of course it's real. There's just absolutely no way to know, even if you made the question a lot more specific. (What does it mean, specifically? What would you expect to see if it were true vs false?)

As real as math

Max Tegmark came up with different "levels" of multiverse, and the first level only depends on the universe (not just observable) being infinitely large in volume - which as I understand it seems likely to be the case. (Certainly hasn't been ruled out at least)

Given infinite space with accompanying infinite matter/energy, there will be infinite repeats and near repeats of the stuff in any given volume of space - the volume of the observable universe for example, meaning us and everything we can interact with (even just see) is repeated infinite times, and with all physically possible variations.

And that's without any particular interpretation of quantum mechanics.

dang, cosmology is weird. I went to reply about the big bang, the age of the universe, and how it can't be infinite, but instead decided to look it up. I really don't understand this stuff.

Not everyone in this comment section uses correct nomenclature. It would be preferable for those bugged by it to use empathy to understand the true meaning of the comments (or, if impossible, to ask politely about the meaning) and to correct the mistakes in a nice way, avoiding the urge to take the wrong words litterally - that doesn't add to the discussion.

Hopefully that can be the case in one of the multiverses. I doubt it will be this one.
You mean I shouldn't react emotionally and impulsively because I disagree and feel personally offended by the first sentence in your comment?

You might enjoy this recent lecture from Neil Turok: https://youtu.be/rsI_HYtP6iU

Over the past decade or so he's been putting out papers with a model that's only a slight change to the standard model - that at the point of the big bang there was an expansion in both directions of time leading to a CPT symmetric universe.

He and his coauthors have managed to address a number of the major outstanding unsolved challenges in cosmology with only this theory, and are perhaps only a few years away from having some of its testable predictions verified. And at least one of the recent CERN projects that would have falsified the idea came up short just the other year.

If he lives long enough, he may well end up with a Nobel for his efforts.

It's probably my favorite bit of theoretical cosmology these days.

According to the many worlds interpretation of quantum physics there's not only alternative universes, but that there's an infinite amount of them including ones exactly like this one with just slight variations.

This video from PBS Space Time is a good summary of the current state of science on this topic. https://youtu.be/z-syaCoqkZA?si=nRK78T_FAD5By-f0

As in the classic _theory idea, that each time a decision is made a new reality is created for each choice? For it to be possible would break our fundamental laws regarding conservation of mass and energy. Where would this alternative universe get all that energy and mass from? It’s possible our current understanding of the law is inaccurate, but it’s extremely unlikely.

why would alternative universes share a single source of energy? couldn't each have their own?
- I’m thinking about the _theory idea that involves a new universe being created every time a decision is made. Like if I flip a coin and get heads, another universe suddenly pops up where I got tails. It would need to create its own energy and mass out of nothing, which would break the laws of conservation.
  I’m not super well versed in multiverse _theories hypotheses though. It is slightly more probable that when our universe began existing, so did all the other alternative universes, in which case they would all have their own energy and mass. But then there would be essentially no chance that we would have alternative selves like in classic multiverse theories, since each universe would have started from the very beginning of time and evolved on its own path.
  Edit: some wording
Where did all the energy in the universe come from? This is not physics, it's methaphysics. We are talking about things that are outside our knowledge and science is descriptive, not prescriptive. Extrapolations must be tested. This is not much different from saying that the most remote galaxies receding at speeds faster than light is impossible, because nothing can travel faster than light. Maybe it's just matter that can't break that rule, while space itself can expand at whatever speed, or maybe it's just an error somewhere in our assumptions. We still don't know. But we DO think they are receding faster than light.
- That's not a very good example I don't think. I'm no cosmologist, but my understanding is that we know pretty well that distant celestial objects are moving away from us faster than the speed of light. Special relativity doesn't really say anything about non local objects, and the expansion of space does not violate locality. No information is being transmitted faster than the speed of light. The medium through which light is traveling is changing. It's not some big unsolved mystery.

I like Neil Degrasse Tyson's take on this one. At first we thought there was only one Sun, then we figured out the sun was a star, like so many others. Then we thought there was only one galaxy, then we figured there are much more than that.

Now we think there is only one universe, but it seems nature isn't very good at creating only one version of something. It's not unreasonable to think there are many universes. We just can't prove/observe it yet.

https://youtu.be/1jmNzlTd09E?si=5w486YFlK4IbqpmK

Watch this^^^^^^

There are different levels of multiverse that are logically possible with our current understanding of physics.

It's incredible to think about.

I honestly think that whatever soup we're all swimming in, it's just one big soup. There are not multiple soups. There are multiple solar systems and multiple galaxies within the soup, but it's just one big universe. We cannot fathom how big the universe is or if it's infinite or even if the universe is an infinitely massive sphere.

That's how I picture it. One big infinitely massive spherical universe.

But If it's infinite, how can it be a sphere?

I dunno, some things are too massive for us to comprehend.

There are multiple solar systems
There's only one solar system in the universe.
Reference: https://science.nasa.gov/solar-system/facts/
- I'm confused, are you saying there's only one solar system because we've only really named our planetary system the "solar system" ?
  Not to be rude but that seems pedantic because there's more than one definition and according to Merriam Webster
  solar system noun: the sun together with the group of celestial bodies that are held by its attraction and revolve around it also : a similar system centered on another star
  Meanwhile, elsewhere on the same site, over here https://spaceplace.nasa.gov/other-solar-systems/en/ says:
  Our solar system is just one specific planetary system—a star with planets orbiting around it. Our planetary system is the only one officially called “solar system,” but astronomers have discovered more than 3,200 other stars with planets orbiting them in our galaxy.
  Genuinely curious to learn as I do not know a lot about this stuff and feel like I am missing something?

In addition to what everyone else has already said, consider this: the "Universe" is by definition everything that exists. There cannot "be" things which do not exist. Therefore the question cannot be answered because it presupposes something could exist which is not part of the universe.

That's a philosophical answer, not a scientific one.
- No, it's a logical answer. The definition of the word universe is "everything that exists"
  Edit: this is really just another way of expressing that the hypothesis isn't testable. If something could be measured which could validate or falsify the hypothesis, then that thing being measured would have to exist, and therefore has to be part of the universe.